What’s scarier than a tyrannosaur? Three tyrannosaurs. That’s simple, undeniable math. The question is whether or not the tyrant dinosaurs ever prowled together in real life. Up until now, the evidence has been equivocal. But a trackway found in British Columbia finally provides firmer ground for speculation on the social lives of these celebrated carnivores.

The idea that large tyrannosaurs – like Albertosaurus and the mighty Tyrannosaurus itself – worked together to bring down prey isn’t new. University of Alberta paleontologist Philip Currie has been championing the hypothesis for over a decade, citing bonebeds that contain multiple individuals of the Late Cretaceous tyrannosaurs Albertosaurus and Daspletosaurus. And three years ago Currie went to the public with his idea in the form of a book and documentary called Dino Gangs. “If dinosaurs hadn’t become extinct, would gangs of killer tyrannosaurs now rule the world?”, the hyperbolic documentary asked.

At the time, I wasn’t sold on tyrannosaurs being communal carnivores. Just because a whole bunch of dinosaurs ended up buried together doesn’t mean that they actually lived and hunted together. A mass of dinosaur skeletons – such as an Albertosaurus bonebed containing 12 individuals – can contain elements from animals that died at different times or were brought into a small area by unusual circumstances. Accumulations of dinosaur bones represent the circumstances of death and burial more than clues about life.

If evidence for social tyrannosaurs exists, it has to be in a less ambiguous form. Tracks hold the most potential.

Tracks and other traces are signs of prehistoric behavior and biology. For example, we know tyrannosaurs fought by biting each other on the face from healed wounds on their skulls. Whether tyrannosaurs lived in groups, though, requires something more. While an Albertosaurus bonebed is ambiguous evidence for social behavior, a trackway showing that tyrannosaurs walked together would be a much clearer sign of social tyrants.

This is what happened with “raptors.” The pack-hunting idea played out in Jurassic Park was based on a Deinonychus quarry that contained multiple predators as well as the apparent prey, but the site only really shows that several of the sickle-clawed predators died at the site. It has only been more recently, with the discovery of distinctive, two-toed tracks that paleontologists have been able to confirm that raptors at least sometimes walked together.

The problem for tyrannosaurs is that their footprint record is very poor. Some supposed tyrannosaur tracks – such as a big, three-toed prints found in a Utah coal mine – turned out to be prints left by shovel-beaked hadrosaurs. Other, authentic tyrannosaur tracks are isolated specimens, recording a single footfall on the Cretaceous ground and nothing more. This historic dearth of trace evidence is what makes the tyrannosaur trackway discovered in Canada so important.

A diagram of the tyrannosaur trackways found in British Columbia. From McCrea et al., 2014.

In October 2011, a few months after Dino Gangs debuted, local outfitter Aaron Fredlund discovered two dinosaur tracks in the roughly 75 million year old rock of northeastern British Columbia’s Wapiti Formation. An excavation by the Peace Region Palaeontology Research Centere turned up a third track in the sequence, but the best find didn’t come until the following summer. In August 2012 paleontologists and volunteers found two more sets of tracks right next to each other.

Unfortunately for paleontologists, the dinosaurs who left the footprints did not die in their tracks. But, as reported by Peace Region Palaeontology Research Centre researcher Richard McCrea and colleagues in a new PLoS One paper, there was only one type of dinosaur alive in the area that could have left such prints. All of the tracks are big – with a length of over 19 and a half inches – and were made by a dinosaur with three forward-pointed toes that ended in sharp claws. The tracks had to be made by a tyrannosaur.

Detail of Trackway A. From McCrea et al., 2014.

Which species of tyrannosaur created the tracks is unknown. Three large tyrannosaurs – Albertosaurus, Gorgosaurus, and Daspletosaurus – all lived in western Canada at the time the tracks were made. But the tracks are distinctive enough that, following trace fossil convention, McCrea and coauthors have given them their own name. The tracks have hence been labeled Bellatoripes fredlundi, an homage to the “warlike foot” shapes of the tracks found by Fredlund.

The hypothesized step cycle for the tyrannosaurs that made the trackways. From McCrea et al., 2014.

While they might superficially look like trident-shaped potholes, the tracks provide a wealth of information about the animals that made them. The animal that made the first-discovered trackway, for example, was apparently missing the end part of the second toe of the left foot, leaving impressions of the nub as it walked. And the details of the footprints showed that these tyrannosaurs walked in a different way than some other carnivorous dinosaurs. Trackways made by predatory dinosaurs typically show that the dinosaur put their foot down and lifted their foot out in a forward motion, while the tyrannosaur tracks suggest that these animals lifted their feet out in a backwards motion.

But the arrangement of the tracks is what has gained the most attention and plays into speculations of tyrannosaur packs. All three trackways face the same direction and were made in close proximity, with 18 feet between Trackway A and B, and eight feet between B and C. While there’s a chance that the trackways were made by three individuals that traveled the same area at different times, McCrea and colleagues consider this unlikely. Tyrannosaurs were a rare part of the fauna, there is no sign of an obstacle that would create a bottleneck requiring tyrannosaurs to walk the same path, and the detail of the tracks – down to foot scales on some – hint that they were all made around the same time, while substrate conditions were consistent. That two, if not three, tyrannosaurs were walking together is the simplest explanation of the pattern.

So did tyrannosaurs hunt in packs? Maybe. The tracks, stunning as they are, can only take us so far.

There remains a shred of doubt that these footprints were made by a single social group. If the tracks showed some sign of interaction between the animals – like raptor footprints that show one adjusting its course to move out of the way of another – then we could be sure. In fact, I’ve encountered this problem out in the field while looking for tyrannosaurs and their Mesozoic kin. Sometimes I’ll find myself closely following the tracks of another fossil hunter, even though they passed by minutes or hours before I did, and I’ll leave my own trail right alongside theirs.

But let’s say the site really does record a social group, as it seems to. We don’t know why the tyrannosaurs were walking together. Was this a family? A hunting party? Suitors trying to follow a mate? Nor do we know how long the band remained together. The tracks only record a few brief moments in Late Cretaceous time when, if even for a moment, tyrants flocked together.

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10 thoughts on “Tracks Hint at the Social Life of Tyrant Dinosaurs”

Oh my God! This is a very crucial paleontological discovery, especially for tyrannosaurs! For now, I believe that the tyrannosaurs could hunt in gangs, but not always! And yet they were far from harmonious, as the bite marks (even healed ones) reveal to science. They might have behaved similarly to crocodiles and komodo-dragons. The best documentary to show the tyranosaur gang hunting without planning or strategy is BBC Planet Dinosaur – Episode 3 – Last Killers. Daspletosaurus was heavier than the two, but it could’ve been juveniles. Now identifying the exact species from 75mya North America, will at best be hardly difficult, or worse, impossible.

I don’t think there’s much that can be extrapolated from this. There are many possibilites. A female and couple suitors seems as good a guess as any. What can be known of Tyrannasaurs society? We can only study extant apex predators and wonder if T Rex followed similar organizational patterns.

There’s no way to know that these animals were walking together… mud is muddy for hours, days, weeks sometimes (and let’s not forget how wet it was back then)… It could be two animals that walked through the mud at different times -or- it could even be the SAME animal walking the same way again (if they had a marked territory, they might do “their rounds” like cats do). While this discovery is very, very cool, I’m not convinced this means they were walking shoulder-to-shoulder as a “pack”.

Is there a reason theoretical correlations to bird behaviors are coming less into play in this discussion? Even cranky loner-type raptors sometimes tolerate a loose flock, for migratory purposes, intolerant crows roost together, and Harris’s hawks do hunt in groups. However, as you say, Brian, there are so many possible behavioral conclusions to be drawn from these tracks, and they could just be situational. Maybe folks will look again at trackways found a while back, for more evidence.

Are we really to believe that birds simply dropped all “pack” behavior as they moved into the Cenozoic? “Pack” hunting is typical of mammals – and that usually occurs through pheromones and their interaction with the mammals’ more sensitive olfactory system. Birds, on the other hand, are noted for hunting via their excellent vision – which comes at the expense of their rather weak olfactory system. Harris’ hawk is the notable exception rather than the rule. The olfactory region of the T. rex brain suggests an emphasis on the olfactory system – but it seems more akin to that of a carrion-eater. At any rate, assessing behavior from a set of footprints is hardly the stuff of science. Those animals may actually have been running *from* something rather than *to* something.

Johnny O: “assessing behavior from a set of footprints is hardly the stuff of science”

Well, there is this field of science called ichnology, you can look it up. Also, assessing behaviour and identity of animals from their tracks is the most science-like activity that large numbers of humans have been conducting for thousands of years, based on inference from incomplete data and constant testing of conjectural hypotheses. It could be argued that the entirety of modern science is just tracking methods applied to different kinds of data.

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Ed Yong is an award-winning British science writer. Not Exactly Rocket Science is his hub for talking about the awe-inspiring, beautiful and quirky world of science to as many people as possible.
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